Chemistry 240Summer 2001

Benzene Structure and also the aromatic Ring
Symbols for the Benzene RingKekule StructuresResonanceHuckel RuleElectrophilic aromatic Substitution -- Mechanism
Today we"ll find that resonance is an extremely important in knowledge boththe structure and the reactions of aromatic compounds. First, let"s takea look in ~ the structural depictions which distinguish fragrant compoundsfrom those that aren"t aromatic.The most frequently encountered fragrant compound is benzene. The usualstructural representation for benzene is a 6 carbon ring (representedby a hexagon) which consists of three double bonds. Each of the carbons representedby a edge is additionally bonded come one various other atom. In benzene itself, theseatoms room hydrogens. The double bonds room separated by single bonds sowe identify the plan as involving conjugated dual bonds. One alternativesymbol supplies a circle within the hexagon to stand for the six pi electrons.Each the these symbols has great and poor features. We"ll usage the three doublebond prize simply due to the fact that it is likewise routinely offered in the text.

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Keep in mind that if the hexagon consists of neither the three dual bondsnor the circle, the link is no aromatic. The is simply cyclohexaneand there room two hydrogens on each carbon atom. This is basic to mistakewhen hurrying, therefore be cautious when you are intepreting any structural formulaswhich include hexagons.The structure with three double bonds to be proposedby Kekule as an effort to describe how a molecule who molecular formulawas C6H6 might be developed out that carbons which makefour bonds. The ring and also the three double bonds right the molecule formula,but the structure doesn"t describe the chemical habits of benzene at allwell. Each of the dual bonds would be supposed to display the characteristicbehavior of one alkene and undergo addition reactions, but this is no howbenzene reacts.In particular, us would mean a carbon-carbon double bond come reactquickly v bromine to do a dibromo compound. This is what alkenes dovery readily, and also in fact it is a helpful test because that alkenes in the laboratory.Benzene does no react v bromine uneven a an extremely bright light or a strongcatalyst is used, and then the reaction is no an enhancement reaction. Weconclude that there is something rather unusual around the double bonds inbenzene.Kekule (thinking around this problem before bonds were taken aspairs the electrons) said that there space two creates of benzene whichdiffer in the places of the dual bonds. His idea was that these werein rapid equilibrium, so fast that over there was never ever a fixed ar forthe twin bond. One could say that an approaching bromine molecule couldnot "find" a twin bond to react with.There were several various other structures proposed forbenzene, yet a much much more satisfactory method became feasible when webegan to understand that covalent bonds consist of bag of electron sharedbetween atoms. The difference between the two structures Kekule envisioned(called Kekule structures) is only the difference between the locationsof 3 pairs that electrons. This is exactly the kind of case whereresonance have to be involved. The hybrid or "average" the the 2 Kekule structureshas one sigma bond and also one-half that a pi bond between each 2 carbon atoms.Thus each carbon is join to each of its neighbors by a one-and-half bond.Each shortcut in the benzene ring has the same variety of electrons and is thesame length. This snapshot is in finish accord v experiments whichshow the all carbon-carbon bond in benzene are the same length, withno note of shorter (double) or much longer (single) bonds. It additionally helps explainwhy benzene does not undergo enhancement reactions: there space no straightforward pibonds.Recall that resonance has another important feature: once resonanceis involved, the actual structure is an ext stable than we would mean fromany of the frameworks we write utilizing the one heat = 2 electrons symbolism.This extra lowering that energy, which because that benzene is around one-third asmuch as making a typical covalent bond, is quite necessary in the reactionsof benzene and other fragrant compounds. Together we will see, reaction of thebenzene ring almost always result in commodities which in i beg your pardon the benzenering stubborn -- an outcome of the stability.When resonance concept was first applied to understandingthe framework of benzene, the vital feature appeared to be a resonance hybridof ring structures containing alternating solitary and twin bonds. Thisimmediately resulted in attempts come make and also study compounds favor cyclooctatetraeneand cyclobutane. These compounds likewise have ring structures with alternatingsingle and dual bonds.Cyclooctatetraene has been made, yet it does not posess the propertiesof extra stability and resistance to addition reactions which distinquisharomatic compounds. It easily adds bromine, for example. Cyclobutadieneis very unstable -- one cyclobutadiene molecule reacts through anothercyclobutadiene molecule instantaneously also at really low temperatures --so it absolutely does no act favor an aromatic molecule and it has actually beencalled "antiaromatic" as a result.It appears that over there is an ext to being aromatic than merely a ring withalternating solitary and double bonds. ~ considerable advancement ofthe basic theory, the pattern which has emerged is that aromatic characteristicsare just expected when there is a ring that pi electron in which the numberof pi electrons is equal to 4n + 2 (where n is one integer,0, 1, 2, etc.). (This is well-known as the Huckel ascendancy after the discoverer.)We can inspect this versus the compounds we have thought about so far: Benzenehas 6 pi electrons (two because that each pi bond) i m sorry is the number we get from4n + 2 if n = 1. Cyclooctatetraene has actually 8 pi electrons, and also there is nointeger "n" which will certainly make 4n + 2 = 8. Cyclobutadiene has actually 4 pi electronsand likewise doesn"t right 4n + 2. Over there are plenty of other examples which supportHuckel"s rule.It is necessary to be certain that the ring of alternating single and doublebonds is complete. If there is one sp3 hybridized carbon in thering, the conditions for fragrant character are not present, and also we donot worry about checking because that 4n + 2. Here"s one example:Another method to watch this is come look in ~ the ns orbitals which combine tomake the pi bonds. If these ns orbitals combine to kind an uninterruptedring together is the case in benzene, then we can go front to use Huckel"s ruleto examine for the proper variety of pi electrons for fragrant character.If the ring of p orbitals is broken by a CH2 (group or anothertetrahedral carbon) through no ns orbital, then the link cannot be aromaticand we need not shot to apply Huckel"s rule.The ns orbitals which make up the unbroken p orbital ring have the right to be associatedwith other atoms 보다 carbon. Two examples are furan and also pyrrole, in whichtwo of the 6 electrons needed come formally indigenous unshared electron pairson oxygen.Such an unshared pair can also come indigenous a carbon atom, which will haveto have a an unfavorable charge. An instance of this is the cyclopentadienideion which deserve to be made by dealing with cyclopentadiene through a middle strongbase. Cyclopentadienide ion is saturated stabilized by its aromaticcharacter that cyclopentadiene (its conjugate acid) has actually a pKaof 16, close to the of water. Cyclopentadiene is a remarkably strong acidfor a hydrocarbon since its conjugate base has the extra stability ofan aromatic compound.Extraordinarily steady cations can likewise be make if your structuresare aromatic. Right here are two:Notice that here the formally positively fee carbon atoms space sp2hybridized and also have an empty ns orbital i beg your pardon completes the cyclic arrangementof ns orbitals.Let"s finish up today by looking at the generalmechanism for the characteristic reactions of fragrant compounds -- electrophilicaromatic substitution. The many important qualities of these reactionsfollow straight from the stability of the aromatic ring. First, these reactionsare generally catalyzed by strong electrophilic (Lewis acidic) catalystslike H2SO4, AlCl3, and also FeCl3which are required to get rid of the stability of the aromatic ring. Second,these room substitution reaction since enhancement reactions would certainly interruptthe ns orbital ring and also destroy the fragrant stability.Even despite the outcome of the attack of electrophiles top top benzene issubstitution fairly than addition, the very first step is the very same as in electrophilicaddition come alkenes -- assault of the electrophile on a pi bond and also theformation of a new sigma bond in between a carbon atom and the electrophile.The carbocation which is developed undergoes lose of the H+ native carbon which was attacked. The electrons from the C-H bond are returnedto the fragrant pi electron ring and also aromatic stability is restored.

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Notice the the intermediate right here is a carbocation, but it is not aromatic.The carbon bearing the hydrogen and also the electrophile is sp3hybridized and also has no p orbital to contribute to a cyclic p orbital system.The carbocation intermediary is somewhat resonance stabilized, though,by a resonance setup which is very similar to the one we experienced in theaddition that electrophiles to conjugated dienes.This intermediate is a carbocation, and also as we will certainly see following time, itsstability is vital in determining how fast the reaction goes and (inbenzene ring which be affected by each other substituents at among the carbons) where theelectrophile attacks. The crucial thing to recognize currently is the the positivecharge and the corresponding carbocation characteristics only appearat positions ortho and also para relative to the allude at whichthe electrophile attack. (Nomenclature is cure in Sec 6.3 of Atkins& Carey.) This will rotate out come be rather important, therefore verify thisfor yourself.Back come the Course synopsis